Modelling of gradient bending magnets for the beam dynamics studies at ALBA

ALBA is a 3^rd generation light source under construction close to Barcelona, Spain. The lattice chosen consist in a DBA-like structure where most of the vertical focusing takes place in the bending magnets, in order to maximize the space allocated for insertion devices in the lattice and to reduce the emittance. In this case the tunes of the storage ring will be strongly affected by the focusing of the magnetic field of the bending magnet. In existing storage rings with gradient bending magnets it has been realized that the real vertical tune of the machine is slightly different from the theoretical one. In order to avoid this for ALBA we investigated the right modelling of the bending magnet. The corresponding procedures are described within this paper. The parameters of the model are estimated from the field map on the magnet midplane computed with 3D simulations.